In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation caused by passage of current through the gas. One class of HID lamps comprises inductively-coupled electrodeless lamps which generate an arc discharge by generating a solenoidal electric field in a high-pressure gaseous lamp fill. In particular, the lamp fill, or discharge plasma, is excited by radio frequency (RF) current in an excitation coil surrounding an arc tube. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, resulting in a toroidal arc discharge in the arc tube.
At room temperature, the solenoidal electric field produced by the excitation coil is typically not high enough to ionize the gaseous fill and thus initiate the arc discharge. One way to overcome this shortcoming is to lower the gas pressure of the fill, for example, by first immersing the arc tube in liquid nitrogen so that the gas temperature is decreased to a very low value and then allowing the gas temperature to increase. As the temperature rises, an optimum gas density is eventually reached for ionization, or breakdown, of the fill to occur so that an arc discharge is initiated. However, the liquid nitrogen method of initiating an arc discharge is not practical for widespread commercial use.
A recently developed starting aid for an electrodeless HID lamp is a gas probe starter, such as that described in commonly assigned, copending U.S. patent application Ser. No. 622,247, of V. D. Roberts et al., filed Dec. 4, 1990, now allowed, which is incorporated by reference herein. The gas probe starter of Roberts et al. includes a fixed starting electrode coupled to a starting chamber which is attached to the arc tube and contains a gas. Preferably, the gas in the starting chamber is at a relatively low pressure as compared with that of the arc tube fill. In the chamber, the gas may be switched between conducting and nonconducting states corresponding to lamp-starting and normal running operation, respectively. In particular, during lamp-starting, a starting voltage is applied to the starting electrode, which causes the gas in the chamber to become conductive. As a result, a sufficiently high voltage is capacitively coupled to the inside surface of the arc tube to break down the gaseous fill contained therein, thus initiating an arc discharge.
A suitable starting circuit for coupling a starting voltage to a gas probe starter is described in commonly assigned, copending U.S. patent application of Cocoma et al., Ser. No. 622,024, filed Dec. 4, 1990, which comprises a resonant LC circuit of variable impedance. Upon application of an RF signal to the excitation coil of the lamp, the starting circuit of Cocoma et al., Ser. No. 622,024, resonates to a sufficiently high voltage to initiate a discharge in the starting chamber which is capacitively coupled to the arc tube, thereby initiating an arc discharge therein. In another suitable alternative starting circuit, as described in U.S. Pat. No. 5,057,750 of G. A. Farrall et al., issued Oct. 15, 1991, the resonant circuit is retuned after initiation of the discharge in the starting chamber in order to ensure that a sufficiently high voltage is applied to the arc tube for initiating the arc discharge, even in relatively low-energy circuits. The Cocoma et al. application and Farrall et al. patent are incorporated by reference herein.
The starting circuits of the above cited references further describe coupling a switch, or a parallel combination of a switch and an additional resonant circuit, in series with the resonant inductor of a Class-D type ballast to ensure suppression of the discharge in the low-pressure starting chamber by detuning the starting circuit after initiation of the arc discharge. By extinguishing the discharge in the starting chamber, the flow of currents between the low-pressure starting discharge chamber and the arc tube, which would otherwise eventually have a detrimental effect on the arc tube wall, is avoided.
Although the hereinabove described circuits for ensuring the suppression of the discharge in the starting chamber are effective, it may be desirable to provide an improved gas probe starter which does not require additional circuitry to extinguish the discharge in the starting chamber.